In the operation of a pump with high head and large capacity, because of the high head and large discharge flow rate, total energy which causes vibration and noise is great. In particular, operation with low noise and low vibration is desired in normal driving points of the pump for obtaining a high efficiency.
1. Occurrence of discontinuity in head capacity curve of pump operation
In a pump operation, it is known that, when the pump is operating under a small flow rate region, a reverse flow of the discharge occurs in the runner on the shroud side of the pump inlet. This phenomenon is a main cause of the discontinuity in the head capacity curve of the pump operation, where sudden changes of flow of the discharge occur at the runner vane entrance. A. J. Stepanoff also proves this phenomenon in his publication: "Centrifugal and Axial Flow Pump" published by John Wiley & Sons, Inc.
In a pump with adjustable guide vanes, the head capacity curve changes in accordance with the opening of the guide vanes, which also changes the range of capacity where discontinuities in head capacity curve occurs. When the operating capacity of the pump is below the range where discontinuities in the head capacity curve appears, the noise and vibration induced during the operation of the pump is much greater than that during the operation above the range of discontinuities. This phenomenon makes it objectionable to operate a pump with high head and large capacity, especially in a starting operation in high static head regions.
2. Conventional starting operation of a pump or a pump turbine by controlling adjustable guide vanes
In order to start a pump operation, it is a known method that first the guide vanes around the pump runner are fully closed under a condition in which compressed air is introduced within the runner casing to depress the water around the runner so that the runner can be rotated by a motor freely in air with minimum losses. When the runner begins to rotate at a synchronous speed of the motor, the guide vanes are gradually opened to a predetermined opening position where the pump is designed to operated with maximum flow rate and efficiency.
However with a conventional starting operation of a high head large capacity pump, owing to discontinuities in head capacity curve of the pump, it is impossible to obtain a desired maximum operation at a corresponding predetermined opening position of the adjustable guide vanes. Worse yet, the conventional operation is accompanied by large noise and vibration not only in a period of the starting operation but also in the normal operation.
3. Proposed counter measures for overcoming the undesirable operating characteristic of the pump
There have been many counter measures for overcoming this problem. For example Stepanoff discloses that the discontinuity in the head capacity curve can be reduced or eliminated by providing two concentric bell shaped baffles at the pump suction to distribute the inlet flow.
By these methods which employ new or improved structures of the pump, head capacity characteristics can be improved only in a small flow rate region, but in a large flow rate region, the pump characteristics get worse. But there has been no counter measure for overcoming the problem by employing only a new or an improved method of starting for pump operation.